Compact diode pumped solid state laser

ABSTRACT

A compact diode pumped laser including a nonuniform, single- or double-sided diode pumped laser head and a polarization output coupled (POC) resonator. The POC resonator employs reflections from two opposing uncrossed roof prism mirrors to produce a uniform near field and far field beam with diffraction or near diffraction limited quality. The single laser head particularly includes a laser rod, a sapphire envelope located about the rod, an area of antireflection coating located on the sapphire envelope between the rod and the diode array, and a high reflectivity nickel-plated indium layer located on the sapphire envelope on the surface thereof outside of the area of antireflection coating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates generally to lasers and, moreparticularly, to a compact, solid state diode pumped laser.

2. Description of Related Art

In previous diode pumped laser designs, especially with rod geometries,a great effort was made to provide uniform diode pumping by locating upto five or more diode arrays around the laser rod, thus creatingfive-sided pumping. With diode pumping, the absorption is high ascompared to flash lamp pumping, since the diode output is in a narrowwavelength range at the peak absorption of the lasing material (forNd:YAG it is 808 nm); a nonuniform inverted energy distribution resultsin the laser rod. By distributing the diode arrays in multiple modulesaround the laser rod, improved uniformity of absorption energythroughout the laser rod is achieved.

Such diode pumped lasers require a complex laser head design ofrelatively large size in order to mount and cool the diode arrays.Furthermore, the distance from the center of the rod to the edge of thelaser head is relatively large. The relatively large distance increasesthe size of the folded resonator since the folded beam must clear theedge of the laser head. For breadboard and commercial lasers, which arenot limited in size, this relatively large folded resonator size doesnot present a problem. But for military systems, small size and weightis critical, and it is in this area that the subject invention providesthe greatest benefit.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to improve lasers;

It is another object of the invention to provide an improved diodepumped laser;

It is another object of the invention to reduce the size and weight ofdiode pumped lasers;

It is another object of the invention to provide a laser head design ofreduced size and weight; and

Still further objects of the invention include improving the beamuniformity, the extraction uniformity, and the beam divergence of alaser.

These and other objects and advantages are achieved according to theinvention by providing a compact diode pumped laser which includes anonuniform, single- or double-sided, diode pumped laser head and apolarization output coupled (POC) resonator. The POC resonator employsreflections from two opposing uncrossed roof prism mirrors to produce auniform near field and far field beam with diffraction or neardiffraction limited quality. This POC resonator is of benefit to anynonuniform gain and thermal lasing distribution created in an activegain medium and placed inside the resonator (for example, one-sidedflash lamp pumping or other means of creating nonuniform gain/thermallasing distributions).

The novel combination of a single- or two-sided pumped laser head with anoncrossed roof prism POC resonator is of reduced size and complexitywhen compared to prior art multiple-sided (>2) diode pumped lasers withflat-flat mirror, flat mirror-roof prism, and crossed roof prismsresonator designs. The efficiency, the beam quality, and the alignmentstability are comparable or superior to the prior art designs.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention, which are believed tobe novel, are set forth with particularity in the appended claims. Thepresent invention, both as to its organization and manner of operation,together with further objects and advantages, may best be understood byreference to the following description, taken in connection with theaccompanying drawings.

FIG. 1 is a perspective view of a single-sided diode pumped laser headaccording to a preferred embodiment;

FIG. 2 is a broken-apart cross-sectional view taken at 2--2 of FIG. 1;

FIG. 3 is a cross-sectional view of a double-sided diode pumped laserhead according to a preferred embodiment; and

FIG. 4 is a perspective schematic view of resonator apparatus employablewith the laser head structures of FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe art to make and use the invention and sets forth the best modescontemplated by the inventors of carrying out their invention. Variousmodifications, however, will remain readily apparent to those skilled inthe art, since the generic principles of the present invention have beendefined herein specifically to provide a particularly compact and usefullaser head and cooperating resonator design.

FIG. 1 illustrates a single-sided diode pumped laser head 11 accordingto a preferred embodiment. The laser head 11 includes a heat exchanger13 mounted atop an aluminum housing 14 including an upper housingportion 16 mounted to a base 23. A pumping diode array 15 (FIG. 2) and acylindrical laser rod 21 are located within the housing 14.

The laser rod 11 is encased in a sapphire thermal conductor envelope 19,which is of annular cross-section with the exception of a flat area 17located just below the diodes of the diode array 15. This area 17 iscoated with an antireflection (AR) coating. The thickness of thesapphire envelope 19 may be, for example, 2 millimeters (mm), while thecoating of area 17 is a very thin dielectric optical coating (<10 μm inthickness).

The remainder of the sapphire envelope 19 is provided with a highreflectivity coating 18. The high reflectivity coating 18 is adielectric coating selected to provide high reflectivity at thewavelength of the pumping diodes 15, e.g., 808 nanometers and lowreflectivity at the wavelength of the coherent radiation emitted by thelaser rod 21, e.g., 1.06 nanometers. Various other coating materialshaving various selected reflectivities may be used, depending on theparticular laser medium and other design parameters, as will be apparentto those skilled in the art.

The high reflectivity coating 18 is preferably provided by a blackindium foil sheet which provides an interface to absorb the1.06-nanometer radiation, while at the same time providing a soft, blackgasket or cushion between the sapphire material 19 and the aluminum ofthe housing base 23. The thickness of the indium foil may be, forexample, 0.010-inch, and such foil is commercially available from TheIndium Corp. of America, 1676 Lincoln Avenue, Utica, N.Y. 13502. Theindium foil is nickel plated in a chemical bath and the surface becomeshighly absorbing at 1.06 nm. To indicate the relative size of the laserhead 11, the dimension d₁ in FIG. 1 is approximately 1.0 inch.

A two-sided pump head 31 is shown in FIG. 3. The pump head 31 of FIG. 3again employs a heat exchanger 33 mounted atop an aluminum housing 35having an aluminum base 37. First and second pumping diode arrays 39, 41are positioned at approximate 45-degree angles to the horizontal, thushaving axes 40, 42 located 90 degrees to each other, and their diodeenergy is directed through respective coating areas 38, 48, whichprovide low reflectivity for 808 nanometers, as in the embodiment ofFIG. 2. The embodiment of FIG. 3 again employs a laser rod 21, asapphire envelope 19, a black indium interface 18, and has a dimension"d₂ " of approximately 1.5 inch.

The laser head shown in FIG. 2 could be cooled by liquid or air and mayhave two-sided pumping as shown in FIG. 3 without significant increasein size and complexity. The configuration of FIG. 2 is used as anexample of the general case, which could include a liquid-cooled laserrod and diode arrays for higher power applications.

The single-sided pumped laser head 11 of FIG. 2 features reduced size.This laser head 11, when compared to prior art multiple-sided pumpedheads, is about 50% smaller in size for the same power output. Thissingle-pumped laser head 11 has a nonuniform energy absorption which ishigher at the edge of the rod 21 located nearest the diode array 15.However, this nonuniformity does not affect the near field or the farfield quality of the laser beam when the laser head 11 is used in anovel uncrossed roof prism POC resonator 70, which will now be describedin conjunction with FIG. 4.

The resonator 70 of FIG. 4 includes a first roof prism 61 located on theoptical axis 69 at one end of the laser rod 21 and a POC polarizer 63located on the optical axis 69 at the opposite end of the laser rod 21.Beyond the polarizer 63 are located, in succession, a Pockels CellQ-switch 65 and a second roof prism 71. The polarizer 63 splits off ordirects a portion of the laser energy out of the resonator 70 to providethe output beam 73.

The resonator structure shown in FIG. 4 bears some similarities to thatdisclosed in U.S. Pat. No. 3,924,201. That patent discloses a laserwhich includes a lasing medium and two Porro prism end reflectors withthe roof line of each prism being at an angle (θ) between about 5-85degrees with respect to the plane of polarization and opticallyperpendicular to each other (α=90°) to provide mechanical stability. Abeam splitter is provided to direct a portion of the energy out of thesystem, the output power being determined by the angle of rotation ofthe Porro prism end reflectors.

The resonator 70 illustrated in FIG. 4 includes two Porro prism endreflectors 61, 71. In the embodiment of FIG. 4, the roof line of eachprism is located at an angle (θ) with respect to the plane ofpolarization for optimum output coupling and are, in contrast to priorart, optically "not" perpendicular to each other (α≠90°). Thisarrangement provides maximum intensity homogenization of the beam in thepresence of nonuniform pumping and gain because rays in all radialpositions are shifted to different radial positions on every pass.Multiple reflections in the resonator 70 therefore sweep out and samplethe gain of the complete aperture of the laser rod 21. The arrangementalso provides homogenization of the phase aberrations in the opticalcomponents in the resonator, including nonuniform thermally inducedlasing of the gain medium.

Test results demonstrate near field and far field beam qualityequivalent to six-sided pumping when the angle between the two rooflines is selected to be α=60°. Higher homogenization has been obtainedin flashlamp pumped lasers by choosing an angle which is not an integralfraction of 360 degrees; that is to say, choosing the angle so no rayever returns to its original position. However, the beam in the nearfield and far field was measured to be of excellent quality in thepresent diode pumped embodiment with α=60 degrees, which illustrates theprinciple of the invention.

One important operational parameter is the far field distribution. Forthe single-sided diode pumping geometry of FIG. 2, it is circularlysymmetric and with a beam divergence of 1.0 milliradian. This beamquality meets designator performance requirements and is comparable tothe performance of four or greater-sided pumping geometries.

In summary, those skilled in the art will appreciate that there are twofeatures of the disclosed design which provide a significant improvementover the prior art when they are combined to form a compact diode pumpedlaser:

1. A highly compact, efficient, single or double diode pumped laser headin a two-part design consisting of a diode assembly and a rod assembly;and

2. An uncrossed double roof prism (POC) resonator with the angle betweenthe two roof lines selected to optimize radial homogenizing symmetry bymultiple reflections (>10 reflections).

In FIGS. 2 and 3, the rod assemblies respectively comprise components17, 18, 19, 21, and 23 (FIG. 2); and 18, 19, 21, 37, 38, 48 (FIG. 3);and the diode assemblies respectively comprise components 13, 15, and 16(FIG. 2); and 33, 35, 39, and 41 (FIG. 3).

It may be noted that, in general, a preferred resonator for use withlaser heads constructed according to the invention may comprise anycombinations of surfaces which are retroreflecting in one or both planesof incidence, the two retroreflectors being oriented relative to oneanother in such a way that the ray optical path does not repeat itselfin the resonator, thereby leading to homogenization. Output coupling maybe brought about with a polarizer as shown in FIG. 4, or by some othermeans, for example, such as a partially reflecting pellicle or beamsplitter, a fold which consists of a partially reflective mirror, or apartially reflective surface on one of the retroreflectors.

Those skilled in the art will thus appreciate that various adaptationsand modifications of the just-described preferred embodiments can beconfigured without departing from the scope and spirit of the invention.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention may be practiced other than as specificallydescribed herein.

What is claimed is:
 1. The laser apparatus comprising:a lasing mediumfor generating a coherent beam of radiation; and an uncrossed doubleroof prism resonator including first and second roof prisms havingrespective roof lines and having said lasing medium located therein,said roof prisms comprising respective reflectors of said resonator,said first and second roof prisms having an angle between theirrespective roof lines selected to provide radial homogenization throughmultiple reflections within said resonator.
 2. The laser apparatus ofclaim 1 wherein the angle between said selective roof lines is selectedto optimize said radial homogenization.
 3. The laser apparatus of claim1 further including an output coupling polarizer and wherein said firstand second roof prisms have the angle between their respective rooflines and the output coupling polarizer optimized with regard to outputcoupling.
 4. The laser apparatus of claim 1 wherein the angle betweensaid respective roof lines is an angle other than 90 degrees.
 5. Thelaser apparatus of claim 1 wherein the angle between said respectiveroof lines is selected to be 60 degrees.
 6. The laser apparatus of claim1 further including a single pumping diode array positioned adjacentsaid lasing medium.
 7. The laser apparatus of claim 1 further includingfirst and second diode pumping arrays positioned adjacent said lasingmedium.
 8. The laser apparatus of claim 7 wherein said first and seconddiode pumping arrays each have an axis, the respective axes beinglocated at an angle of 90 degrees with respect to one another.
 9. Thelaser apparatus of claim 6 wherein said lasing medium includes a laserrod and said laser apparatus further comprises:a sapphire envelopelocated about said rod; an area of antireflection coating locatedbetween said rod and said diode array; and high reflectivity coatingmeans located on said rod outside of said area.
 10. The laser apparatusof claim 9 wherein said high reflectivity coating means further providesa cushion between said sapphire envelope and a metal housing base. 11.The laser apparatus of claim 9 wherein said high reflectivity coatingmeans includes indium.
 12. The laser apparatus of claim 9 wherein saidhigh reflectivity coating means comprises a nickel-plated indium sheet.13. The laser apparatus of claim 1 further including means for directinga portion of the laser energy out of said system.
 14. The laserapparatus of claim 9 further including a Pockels cell Q-switchpositioned between said first and second roof prisms.
 15. The laserapparatus comprising:a lasing medium for generating a coherent beam ofradiation; and a resonator having said lasing medium located therein andcomprising any two combinations of surfaces which are retroreflecting inat least one plane of incidence, the two retroreflectors being orientedrelative to one another in such a way that the ray optical path does notrepeat itself in the resonator, with resultant homogenization of saidbeam.
 16. A compact diode pumped laser comprising:a lasing medium; asingle pumping diode array positioned adjacent said lasing medium; and aresonator comprising an uncrossed double roof prism resonator includingfirst and second roof prisms having respective roof lines, said roofprisms comprising respective reflectors of said resonator, said firstand second roof prisms having an angle between their respective rooflines selected to provide radial homogenization through multiplereflections within said resonator.
 17. A compact diode pumped lasercomprising:a lasing medium; a single pumping diode array positionedadjacent said lasing medium; and a resonator comprising any twocombinations of surfaces which are retroreflecting in at least one planeof incidence, the two retroreflectors being oriented relative to oneanother in such a way that the ray optical path does not repeat itselfin the resonator, resulting in homogenization of the output beam of saidlaser head.